Isoprene polymer, in the presence of certain cross-linking photoinitiators, will cure to a smooth rubber and highly chemically resistant framework. This cured polymeric material is used to produce patterns (masks) which become the basis for depositing microcircuits in semiconductor manufacturing. At the completion of the process, the mask is removed utilizing the novel stripping composition of the invention. Upon contact with the stripper, the cured polymeric mask will chemically breakdown, and in turn, may be readily rinsed away.
During the manufacture of semiconductor microcircuits, various inorganic substrates such as single and polycrystalline silicon, hybrid semiconductors such as gallium arsenide, and metals, are coated with a polymeric organic substance which forms a resist mask after undergoing a photolithographic process. The polymeric resist mask is used to protect selected areas of the substrate surface, e.g. silicon, silicon dioxide, or aluminum, etc., from the action of chemicals in both wet (solution) and dry (plasma) forms. The exposed areas of the substrate may carry out a desired etch (removal) or deposition (addition) process. Following completion of this operation and after subsequent rinsing or conditioning, it is necessary that the resist mask and any application post-etch residue be removed to permit essential finishing operations. Upon removal of the mask, specific micro-etched or deposited patterns are left behind. The masking and patterning processes are repeated several times to produce layered microcircuits that comprise the final semiconductor device. Each step requires complete resist stripping and cleaning, to ensure that the final form device is produced at good yields and performs satisfactorily.
To fully appreciate the challenges in removing such materials, it is important to understand the nature of the organic coatings and how they are used in semiconductor manufacturing processes. Organic masking agents comprise many sorts of photoresists. One of the more tenacious polymers is a negative-tone variety that is hydrophobic (non-polar), described as a biazide/cyclized isoprene (rubber) system. Cyclized isoprene is preferred over conventional natural rubber and other polymerized isoprenes due to its more rigid character and reduced solubility. The isoprene polymer is linear, a product of straight-chain Ziegler-Natta polymerization, making it a good candidate for between-chain crosslinking. The isoprene photoresist will react to light and initiate a photochemical reaction. Upon exposure to ultraviolet light of a specific frequency, the azide undergoes free-radical generation and crosslinks between the isoprene chains. The result is a rigid polymer network that incorporates the azide crosslinker between the chains.
The exposed system is less soluble than the unexposed material. The unexposed material is dissolved and rinsed away (developed) from the exposed, leaving behind a negative image as compared to the pattern in which light had traveled. When viewing the remaining pattern under a high resolution microscope (i.e. scanning electron microscopy, SEM), the resultant sidewall of the resist is commonly not vertical (i.e. 90°) from top to bottom. In fact, the pattern wall has a negative slope (i.e.<90°), as measured from the bottom plane of the developed area. This results during imaging when a reduced efficiency of the photochemical reaction (crosslinking) is exhibited as light proceeds downward through the resist, causing less and less of the resist to be imaged and cured. At the pattern edge, the resist near the top surface may be fully cured, yet curing of the material near the bottom is reduced. As a result, the material at the top of the profile has a reduced solubility, whereas that near the bottom is more soluble. During the development process, more of the soluble material near the bottom (substrate) is removed. The resulting pattern is viewed to be larger at the top than at the bottom, giving the effect of a “negative” slope.
This negative slope is needed for depositing thick metal lines in a process commonly referred to as deposition and “lift-off.” Following the patterning process, metal is coated onto the pattern either by plasma deposition or wet chemical plating. After deposition, the resist is stripped from the surface bringing with it the unwanted metal that was originally deposited directly onto the resist pattern. This occurs by a solvent stripping process whereby solvent molecules penetrate exposed resist from the side at the negative slope profile. As the solvent penetrates, the resist begins to swell and dissolve, causing the unwanted metal to “lift-off.” Once the metal and resist enters the bulk chemical, it can then be filtered and reused repeatedly. After the resist is stripped and metal is lifted off and rinsed away, what is left behind are the metal lines that were originally deposited within the resist pattern.
Reliability issues may arise in a lift-off process, or for that matter, any resist strip process, due to the variability in exposure conditions. If this variability is due to factors that affect the curing process, it will result in a change of the chemical make-up of the resist. The factors that control a curing process include light, temperature, and oxygen. For purposes of this description, the focus will be limited to temperature, one of the most common variables in a manufacturing process. Temperature changes may be due to variability in substrate conductivity or thermostat controls when using a hotplate or an oven. An organic material exposed to different temperatures may exhibit varying densities in its bulk form and show changes in surface composition. This is observed in oven-cured polymers where a material coating is heated by convection.
It is generally observed that polymers exposed to convection heat will cure to a higher extent due to the formation of a surface “skin.” The surface skin results from direct contact with heat in the environment (i.e. convection heat), causing accelerated curing to form a higher bulk density polymer at the surface (i.e. skin). This polymer skin will commonly solvate much slower than a material that is cured internally or at lower temperatures. Accordingly, temperature variation is a common process variable, which may produce coatings, which exhibit a range of solubility characteristics. Strippers that are designed to solvate polymers exposed to temperature extremes will be robust for general cleaning processes. This invention describes a robust chemical stripper designed to dissolve and remove fully cured negative-tone isoprene photoresists.
Another chemically resistant polymer used for semiconductor manufacturing is based upon the resin, bisbenzocyclobutene (BCB). This resin is used as a planarizing dielectric for packaging and protecting final form semiconductor wafers. As it is known in the industry, BCB is manufactured by DOW Chemical Company under the tradename, Cyclotene®. Like most resists, it is spin-coated onto a wafer and heat cured. To reach acceptable curing conditions, BCB is oven heated in an inert atmosphere to temperatures reaching the order of 350° C. Applications of the BCB polymer include the protection of multilayer interconnects on GaAs (Gallium Arsenic) devices, a supporting structure for wafer bumping and packaging, and as a dielectric for circuit boards.
Occasionally, BCB polymer must be reworked (removed) by a chemical stripping process. Although it is disclosed that fuming nitric acid may strip the cured BCB material, mineral acids are deemed too aggressive to metals and are considered unacceptable for reworking final form devices, wafers, and boards. Although uncured BCB may be removed by heated organic solvents, the full-cure version has been regarded as being completely impervious to similar chemistries. By a heated chemical operation that is similar in scope to a common photoresist stripping process, the system of the invention is effective in dissolving and removing the fully cured BCB polymer. The invention is observed to be safe for metals as measured during given exposure times.
The common method used in removing cured negative-tone isoprene resist masks or BCB polymer from the substrate is by direct contact with an organic stripper. The chemistry of the stripper penetrates the polymer surface and causes it to swell, whereby the presence of an organic acid undergoes a reaction to hydrolyze and sever the cross-linked portions. As this process continues, more and more of the polymer is exposed until the products of hydrolyzation and dissolution are broken down into relatively small chains that can be filtered and removed from the chemistry.
The known prior art stripping compositions have usually been less than satisfactory or have the distinct disadvantage of presenting unacceptable toxicity and/or pollution problems from the disposal of such compounds as phenol, cresol, and chlorinated hydrocarbons. Other known prior art for removing polymeric organic substances that include inorganic compounds are not suitable such as, aqueous sulfuric acid compositions containing a significant amount of fluoride ion to reduce metallic dulling and corrosion, as exemplified in U.S. Pat. No. 3,932,130. Some photoresist strippers require the presence of fluoride ion stabilizers to prevent metallic corrosion and operate at elevated temperatures. Although these strippers may provide value to industrial applications, they are deemed to be too aggressive for semiconductor devices.
The efficiency and selectivity of a stripper is also desirable. There is a need, accordingly, for improved stripping compositions, which will remove the polymeric organic substance from the coated inorganic substrate without corroding, dissolving or dulling the surface of the metallic circuitry, or chemically altering the inorganic substrate.
This invention aids in semiconductor manufacturing by stripping full-cure (i.e. 200C exposure) negative-tone isoprene photoresists at processing temperatures equal to or below 70° C. within a time period of 15 minutes. The invention offers a benefit over prior art, as disclosed in U.S. Pat. Nos. 4,165,294, 4,992,108, and 6,261,735, where the removal of similar isoprene resist coatings required elevated processing temperatures of>100° C.,>80° C., and>85° C., respectively. Further, the isoprene samples used in this prior art were cured to lower temperatures (i.e. 150C max). The present invention thus affords a substantial advance that benefits industry.
Additionally, the invention fulfills a need for a safe chemical stripper for BCB coatings, which have experienced a range of cure conditions through complete polymerization (i.e. full cure). When the invention is compared to the systems of U.S. Pat. No. 4,165,294, 4,992,108, and 6,261,735, the chemistries described were found to have no observed effect on full-cure BCB coatings. Whereas, this invention was found to remove full-cure BCB coatings within 45 min of exposure at processing temperatures near 110C. Accordingly, the invention provides an improved means to dissolve and remove both full-cure negative-tone isoprene resist and BCB coatings, which have been cured from a limited level to full-cure. By adjustment of the type of sulfonic acid present, removal selectivity between cured isoprene and BCB may be achieved.
It is, accordingly, the objective of this invention to provide a material and process which is employed to thoroughly and selectively penetrate and dissolve fully-cured isoprene negative-tone resists and BCB coatings.
The invention in essence provides an organic stripping composition and system for dissolving cured negative-tone isoprene-based photoresist and BCB coatings. The system of this invention operates effectively without the introduction of toxic substances, operates at moderate temperatures, and is deemed safe to adjacent metals. By adjustment of the type of sulfonic acid present, removal selectivity between the isoprene and full-cure BCB may be achieved. The utility of the invention is particularly advantageous for semiconductor fabrication lines where rapid processing at low temperatures and using a simple alcohol or water rinse is effective for producing clean substrates.